Not everyone knows how to play in a band, take piano for instance, most people solo well but there are a lot of people that just can not accompany a band.
I do not think they should be told not to play but help them to play with the band, practise with them. Most people do not just pick up an instrument and know how to do everything.
I think there are songs that sound great with just piano and some that sound great with the whole band. Vary the music and let them play a part that can be soloed and if they are hindering the band when you try to play together then the pastor or music leader should talk to them.
I do not think u should just kick them out as this would not encourage them to grow musically or spiritually. As a church if u r going to keep someone from playing because they r not good enough to play along then I believe there should be an effort to help build their skills by having practise sessions where they are given pointers and tips and the oppertunity to practice with the band. That is a vital skill that needs to be learned and by teaching it then God will recieve the glory, who knows how God will use this person if they learn this skill and then everyone will be blessed because everyone that took part in teaching this person will be rewarded by God.
It is my opinion that we should build up; not destroy. Pray and seek God for wisdom on the matter and at the same time seek the Word on this issue. Check out some of the proverbs, psalms, and even ecclesiastes. The wisdom books of the Bible are very eye opening at times like these.